1. Field of Invention
The invention relates to a process for the production of electrolyte capacitors having a low equivalent series resistance and low residual current for high nominal voltages, electrolyte capacitors produced by this process and the use of such electrolyte capacitors.
2. Description of Related Art
A commercially available solid electrolyte capacitor as a rule comprises a porous metal electrode, an oxide layer on the metal surface, an electrically conductive solid which is incorporated into the porous structure, an outer electrode (contacting), such as e.g. a silver layer, and further electrical contacts and an encapsulation.
Examples of solid electrolyte capacitors are tantalum, aluminium, niobium and niobium oxide capacitors with charge transfer complexes, or pyrolusite or polymer solid electrolytes. The use of porous bodies has the advantage that because of the high surface area a very high capacitance density, i.e. a high electrical capacitance over a small space, can be achieved.
π-Conjugated polymers are particularly suitable as solid electrolytes because of their high electrical conductivity. π-Conjugated polymers are also called conductive polymers or synthetic metals. They are increasingly gaining economic importance, since polymers have advantages over metals in respect of processability, weight and targeted adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), a particularly important polythiophene which is used industrially being poly-3,4-(ethylene-1,2-dioxy)thiophene, often also called poly(3,4-ethylenedioxythiophene), since it has a very high conductivity in its oxidized form.
Technical development in electronics increasingly requires solid electrolyte capacitors having very low equivalent series resistances (ESR). Reasons for this are, for example, falling logic voltages, a higher integration density and increasing cycle frequencies in integrated circuits. Furthermore, a low ESR also lowers energy consumption, which is particularly advantageous for mobile battery-operated uses. There is therefore the desire to reduce the ESR of solid electrolyte capacitors to as low a value as possible.
European Patent Specification EP-A-340 512 describes the preparation of a solid electrolyte from 3,4-ethylene-1,2-dioxythiophene and the use of its cationic polymers, prepared by oxidative polymerization, as a solid electrolyte in electrolyte capacitors. Poly(3,4-ethylenedioxythiophene), as a substitute for manganese dioxide or charge transfer complexes in solid electrolyte capacitors, lowers the equivalent series resistance of the capacitor due to the higher electrical conductivity, and improves the frequency properties.
A disadvantage of this process and similar processes which use a chemical in situ polymerization is that no solid electrolyte capacitors of low ESR and low residual current which have a high nominal voltage can be produced with them.
After deposition of the polymer solid electrolyte, the oxide layer of the capacitor must conventionally be re-formed in order to achieve low residual currents, as described e.g. in EP-A 899 757. For this, the capacitor is impregnated in an electrolyte and exposed to an electrical voltage which corresponds to the anodizing voltage of the oxide film.
In the production of polymer electrolyte capacitors having nominal voltages of from 16 V, the re-forming of the oxide layer becomes more difficult as the nominal voltage increases, and can no longer be carried out for nominal voltages of from 25 V without seriously impairing the ESR. Weaker re-forming, i.e. a reforming far below the anodizing voltage, remains as the way round the problem. However, this leads to a reduced reliability of the capacitor.
The break-through voltage of the capacitor is a measure of the reliability. The break-through voltage is the voltage at which the dielectric (oxide layer) of the capacitor no longer withstands the electrical field strength and electrical discharges occur between the anode and cathode, which leads to a short circuit in the capacitor. The higher the break-through voltage, the better the quality of the dielectric and therefore the more reliable also the capacitor. The higher the break-through voltage of the capacitor, the higher the nominal voltage at which it can be employed.
In polymer capacitors of low nominal voltage, the break-through voltage is close to the anodizing voltage and therefore far above the nominal voltage, which is typically two to four times lower than the anodizing voltage. In polymer solid electrolyte capacitors of high nominal voltage, however, the break-through voltage drops to significantly below the anodizing voltage due to the problems described above during the re-forming. As a result, the reliability of these capacitors decreases. It is therefore desirable to increase the break-through voltage and therefore the reliability of polymer solid electrolyte capacitors.
Numerous field of use in electronics, such as, for example, automobile electronics or voltage filtering in mains components, require the use of solid electrolyte capacitors of high nominal voltage and low ESR and residual current with a high reliability.